1. Field of the Invention
This invention relates to a direct current power supply deriving a direct current output from an alternating current input and more particularly to a direct current power supply converting alternating current to direct current and connectable directly to household current with various rated voltages.
2. Description of the Prior Art
Today, a power supply system with values of various voltages etc. in countries of the world has not yet been united. The countries are using commercial current with various rated voltages in accordance with backgrounds of their histories, societies and economies.
For example, the rated voltage of a household current is frequently selected at the range of 100 V-120 V for 100 V-system power supply systems or 200 V-240 V for 200 V-system power supply systems. In addition, the domestic power supply system in a single country possibly uses a mixture of the 100 V-system and 200 V-system power supply systems.
Thus, a direct current power supply system for electronic devices is required which can flexibly cope with various voltages of commercial current. A switching power supply is known as a direct current power supply which can cope with two kinds of high and low voltages.
As well known, a switching power supply directly rectifies a commercial alternating current into direct current, without a power transformer, and then converts the direct current into a high-frequency alternating current through an inverter, and then reduces the voltage of the high-frequency alternating current to a desired value through a high-frequency transformer, whose output is rectified and smoothing circuit to provide the required DC output. A power transistor regulates the voltage of the direct current output by means of pulse width modulation.
Since this switching power supply can readily cope with wide variations of the alternating current input voltage and shows a high efficiency, the switching regulator system can handle various voltages. However, the switching regulator system entails a problem in that the action of the switching power transistor causes a switching noise which includes not only a switching-frequency component but also wide noise components up to a high-frequency band and radio frequency band. Therefore, the switching regulator system causes problems on devices which tend to be affected by noises from a radio receiver and the like.
In addition, a method using a direct current power supply system comprising a voltage-reducing power transformer, a rectifying-and-smoothing circuit and a series regulator is known as means for handling the different voltages.
As well known, the series-regulator-type direct current power supply system steps down the voltage of a commercial power supply into a desired value through a power transformer and then rectifies the resulting current into a direct current. The output voltage of the resulting direct current is stabilized by the voltage drop between the collector and emitter of a power transistor of a main line so that no switching noises are produced. However, this system necessarily produces a power loss constituting a product of the voltage drop between the collector and emitter of the power transistor and a load current. The voltage drop to be burdoned on the power transistor and the power loss increase as the input voltage increases. Thus, a problem of heat radiation becomes more important.
Consequently, this system cannot handle the wide variations of the alternating current input voltage in principle and has conventionally provided a primary winding of a power transformer with a plurality of taps and a selector switch, a corresponding selection being manually carried out in response to various rated voltages of a commercial power supply.
The use of the conventional switching-regulator-type power supply system as a direct current power supply system of a radio receiver and the like is difficult because of the occurrence of the switching noises. In addition, this system is complicated and costly. The series-regulator-type direct current power supply system includes a type which provides the primary winding of the input power transformer with the various taps and with the selector switch in order to handle different voltages, the selector switch being manually operated in response to the rating of input voltages, and another type which, without the selector switch, requires the circuit design affording allowances for parts rating and heat-radiation design and allowing an increase in power loss and does not have a selector switch, so as to handle both the higher-voltage and lower-voltage cases of the input power supply. Voltage selector type incurs increases in cost, size and weight and a prominent decrease in efficiency. Thus, the use of the latter type is not reasonable. The former type requires a troublesome actuation of the selector switch and may possibly damage devices due to a misoperation of the selector switch.